Okay, I changed the redirect URL to http://localhost and tried it again.
https://id.twitch.tv/oauth2/authorize?client_id=<my_client_id>&redirect_uri=http://localhost/&response_type=code&scope=channel_editor
I get to this page:
Du bist dabei, Twitch zu verlassen.
Twitch hat keine Kontrolle über den Inhalt und die Sicherheit von http://localhost.
Continue
You are about to leave Twitch.
Twitch has no control over the content and security of http: // localhost.
Continue
When I clicked “Continue” I came to a page where again the following error;